1. Field of the Invention
This invention applies to semiconductor laser integrated circuits in which rows of lasers alternate with reflector strips, this assembly producing laser beams perpendicular to the general plane of the integrated circuit. More precisely, the invention is a system to distribute electric power to the lasers in a row.
The invention is also a method of producing the system to distribute electrical power to a laser integrated circuit.
2. Discussion of the Background
Semiconductor laser integrated circuits are known to the prior art, for example in French patent No. 88 14799, dated Nov. 15, 1988, the appended FIG. 1 being a three-quarter perspective view, taken from the above-mentioned patent and using the same identification numbers.
A laser integrated circuit includes a substrate 27 in category III-V material: to simplify the explanation, it will be assumed that the substrate is in semi-insulating GaAs. This substrate 27 is machined to produce reflectors 28 in the form of parallel strips, trapezoidal in cross-section with the sides inclined at 45.degree. or semi-circular in cross-section. At least flanks 29 of the strips are metallized to act both as 45.degree. -inclined or semi-cylindrical mirrors and electrical conductors.
Semiconductor laser rows 30 are placed between these reflecting strips. Each row comprises several epitaxial layers of semiconductor material, jointly indicated by reference 16. A series of parallel grooves 17, perpendicular to the center line of each row 30, is then machined in these layers by ion etching. Each pair of grooves 17 delimits an emitter ribbon 18 for an individual laser.
Of the series of epitaxial layers 16, at least the first deposited layer 40 is doped to be conductive and overlaps the free surface 31 of the semi-insulating substrate 27 and the flanks 29 of reflector strips 28.
A metallized film 32 covers the entire top free face of each row of lasers 30 while a metallized film 33 covers the entire top face of each reflector strip 28: these two metallized films 32 and 33, combined with the epitaxial conductive layer 40, distribute electric current to the individual lasers.
When the integrated circuit is switched on, each individual laser emits, from its ribbon 18, a coherent light beam 35 via one cut face and a beam 36 via the other cut face. These two beams are reflected, virtually perpendicular to the general plane of the integrated circuit, by the metallized flanks 29 on reflector strips 28. The complete integrated circuit therefore emits a number of parallel beams 35 and 36, perpendicular to the general plane of the substrate.
FIG. 2 is a cross-section of one region of the integrated circuit shown in FIG. 1. It is intended only to illustrate the distribution of electricity to one row of lasers and only shows the outline of one row 30 and two reflector strips 28 since details of the material layers do not affect the electrical distribution problem.
Electricity is fed vertically to a laser emitter ribbon 18, the current flowing from metal film 32 through the epitaxial layers 16 and returning to metal film 33 via the conductive layer 40 on the free face 31 of the substrate and the surface of mirrors 29. The path followed by the current can be reversed, depending on the polarity applied.
However, if we consider the distribution of the current along a row of lasers 30, it can be seen that metal films 32 are parallel to the longitudinal center line of rows 30; in other words they are perpendicular to the center lines of emitter ribbons 18. Although, in theory, the current is distributed to all lasers in parrallel and the conditions applied to each laser are, therefore, identical, practice shows that lasers further from the power supply terminal emit less light because they receive less electrical power. Regardless of the source impedance, the metallized film 32 is so fine that it offers high resistance and the scale of integration in microelectronics makes it impossible to reduce this resistance by increasing the width or thickness of metallized film 32. Consequently, the 10 to 30 lasers in a row do not all receive the same power supply.